Projection objective and method for its manufacture

ABSTRACT

A method of manufacturing a projection objective ( 22 ) of a microlithographic projection exposure apparatus ( 10 ) is described. The projection objective ( 22 ) comprises at least one mirror (M 1  to M 6 ) that each have a mirror support ( 241  to  246 ) and a reflective coating ( 26 ) applied thereon. First imaging aberrations of a pre-assembled projection objective are measured. Before the coating ( 26 ) is applied, the mirror supports ( 241  to  246 ) are provided with a desired surface deformation ( 34 ). If the mirrors (M 1  to M 6 ) are not reflective for projection light without the coating ( 26 ), measuring light is used that has another wavelength. Alternatively, two identical mirror supports ( 246 ) may be provided. One support having a reflective coating is part of the pre-assembled projection objective whose imaging aberrations are measured. The other support is provided with surface deformations before coating and mounting the support into the objective.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention quite generally relates to projection objectives ofmicrolithographic projection exposure apparatus, which have at least onemirror with a mirror carrier and a reflective coating applied thereon.In particular, the invention relates to EUV projection objectives thatuse projection light with a wavelength in the extreme ultravioletspectral range (EUV).

2. Description of Related Art

Microlithographic projection exposure apparatus, as used formanufacturing integrated electrical circuits and other micro-structuredcomponents, reproduce structures, which are contained in a reticle,generally reduced on a photo-sensitive layer that can be applied on asilicon wafer for example.

One of the main aims in the development of projection exposure apparatusconsists of being able to lithographically define structures withincreasingly smaller dimensions on the photo-sensitive layer. Theproduction of particularly small structural sizes requires a lowresolution of the projection objectives. Since the resolution of theprojection objectives is proportional to the wavelength of theprojection light, successive generations of such projection exposureapparatus use projection light with ever shorter wavelengths. Futureprojection exposure apparatus still presumably use projection light witha wavelength which lies in the extreme ultraviolet spectral range (EUV).To be considered here are in particular wavelengths between 1 nm and 30nm and more particularly the wavelength 13.5 nm

When using projection light with such short wavelengths, no sufficientlytransparent materials are available for manufacturing lenses and otherrefractive optical elements. Therefore EUN projection objectives aresubstantially composed of mirrors. The mirrors consist of a mirrorsupport, which for example can be manufactured from a glass blank andwhich has a surface that is manufactured with high precision. Since themirror support for the projection light is almost 100% absorbent andtherefore does not reflect any projection light, a reflective coatingwith a reflectivity in the order of approximately 60% to 70%, is appliedon the surface that is exposed to the projection light.

Coated mirrors however are also used in projection objectives that aredesigned for longer wavelengths. In order to avoid chromaticaberrations, for example, projection objectives that are suitable forthe wavelength 157 nm frequently have a catadioptric structure. Thismeans that they also contain at least one mirror as well as refractiveoptical elements such as lenses.

Due to the small size of the structures to be reproduced, high demandsare made on the reproduction properties of the projection objective.Aberrations can therefore only be tolerated to an absolute minimum.

Generally aberrations are divided into the following two categories. Onthe one hand there are aberrations which result from the design of theprojection objective, i.e. from the specification of the dimensions,materials and distances of the optical elements contained in theprojection objective. These design errors will not be regarded in thefollowing.

On the other hand there are aberrations that are due to manufacturing ormaterial defects and generally can only be corrected properly on thecompleted projection objective. In the case of mirrors for projectionobjectives form errors represent the most important manufacturingdefects. The term “form error” relates quite generally to deviations ofan actual optical surface from its desired form

In order to correct such manufacturing- or material-related aberrations,U.S. Pat. No. 6,266,389 B1 proposes, in connection with an EUVprojection objective, that the finally assembled and adjusted projectionobjective is measured and then the surface of one or several mirrors isreprocessed in such a way that certain imaging properties of theprojection objective are improved. Here, the reprocessing takes place insuch a manner that material is removed locally from the coating of themirrors and/or material is applied locally by laminating onto thecoating.

However, it has been shown that the reflectivity of the coating as aresult of the local reprocessing substantially changes—and to be moreprecise—generally decreases. Although the known manufacturing processallows to reduce wavefront errors, the uniformity of the light intensitydistribution in the image plane of the projection objective may, to anintolerable extent, deteriorate due to the locally alteredreflectivities of individual mirrors.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved methodof manufacturing EUV projection objectives comprising a plurality ofmirrors.

It is a further object of the present invention to provide an improvedmethod of manufacturing EUV projection objectives that allows to reducemanufacturing-related aberrations without significantly changing thereflectivity of the mirrors.

This and other objects of the invention are achieved, according to afirst aspect of the invention, by a method comprising the followingsteps:

-   -   a) providing a mirror support for the at least one mirror:    -   b) assembling and adjusting the optical elements including the        mirror support for the at least one mirror into a housing of the        projection objective;    -   c) obtaining measured data relating to at least one image        property of the projection objective using measuring light,    -   d) comparing the measured data with desired values;    -   e) defining a surface deformation on the mirror support of the        at least one mirror such that an improvement of the at least one        image property can be achieved;    -   f) producing the surface deformation defined in step e) on the        surface of the mirror support;    -   g) applying the coating onto the surface, which has been        reprocessed in step f), of the mirror support;    -   h) assembling the mirror support coated in step g) into the        housing.

According to a second aspect of the invention the above stated object isachieved by a method comprising the following steps:

-   -   a) providing a master mirror support for the at least one        mirror;    -   b) providing a substantially identical copy of the master mirror        support;    -   c) applying the coating onto the master mirror support;    -   d) assembling and adjusting the optical elements including the        master mirror support coated in step c) into a housing of the        projection objective;    -   e) obtaining measured data relating to at least one image        property of the projection objective;    -   f) comparing the measured data with ideal values;    -   g) defining a surface deformation on the mirror support of the        at least one mirror such that an improvement of the at least one        image property can be achieved:    -   h) producing the surface deformation defined in step g) on the        copy of the mirror support;    -   i) applying the coating onto the copy of the mirror support        reprocessed in step h);    -   j) replacing the mirror support assembled in step d) into the        housing by its copy coated in step i).

In this context local aspheric deformations of the surface in themanometer and sub-nanometer range will be referred to in short as“surface deformations”. In the literature such surface deformations arealso referred to as “nanometer aspheres”, see, for example, the essay ofC. Hofmann et al. entitled “Nanometer Aspharen: Wie herstellen undwofür?”. Feinwerktechnik und Meβtechnik 99 (1991), 10, pages 437 to 440.which however refer to the reprocessing of lenses and other refractiveoptical elements.

Thus, in accordance with the invention it is not the sensitive coatingof one or several mirrors, which is reprocessed in order to improve theimaging properties, but always the uncoated mirror support. Therefore nolocal removal of the coating, which could reduce the reflectivity of thecoating, takes place.

Since the surface deformation is produced before the coating is appliedonto the mirror support, there is also no necessity to partly againremove a coating, which is already applied, in order to produce thedesired surface deformation on the then exposed area on the surface ofthe mirror support and subsequently to again cover this area with a newcoating.

In the case of EUV projection objectives, the uncoated mirror supportsreflect practically no projection light at all. Therefore with EUVprojection objectives the measurement is performed according to thefirst aspect of the invention with another, preferably longerwavelength, in which case the mirror supports also have such highreflectivity so as to facilitate measurement of the projectionobjective. In the case of catadioptric projection objectives, which aredesigned for projection light having a longer wavelength, the projectionlight itself may also be used for measurement.

According to the second aspect of the invention, the measurement of theprojection objective is performed with the later used projection lightwith the mirror support being coated. Then, however, not a mirror thatis assembled in the measured projection objective is reprocessed, but astill uncoated, substantially identical copy of the same. This copyshould correspond as accurately as possible with the measured mirror.Since, however, due to manufacturing tolerances no mirrors can be madethat are perfectly identical with mathematical accuracy, the copy canonly be substantially identical to the measured mirror.

Preferably not several mirrors, but just a few or even only one mirrorare reprocessed on the surface of the mirror supports for the correctionof aberrations. In the case of the first aspect of the invention thatpreferably uses measuring light Faith a wavelength other than the laterused projection light, therefore all mirrors, which are not to besubjected to later reprocessing, can already be provided with thecoating before the measurement. This is particularly advantageous, sincethen only the mirror or mirrors to be reprocessed have to be removedfrom the projection objective after the measurement and reassembled intothe projection objective after being reprocessed and coated. The idealvalues, with which the measured data obtained during the measurement arecompared, are therefore to be determined depending on which of themirrors assembled into the projection objective support a coating.

Of course, it also is possible in this context to reprocess andre-assemble into the projection objective a substantially identical copyof an uncoated mirror support that has been assembled into theprojection objective during the measurement.

If, in accordance Keith the second aspect, the surface deformation isproduced on a substantially identical copy of the measured coated mirrorsupport, systematic errors, as they arise for example in optical testingfacilities for checking the dimensional accuracy of surface deformationsproduced, can therefore be compensated, since these systematic errorsoccur with the master mirror supports in the same way as with theircopies.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present invention may be morereadily understood with reference to the following detailed descriptiontaken in conjunction with the accompanying drawing in which:

FIG. 1 is a schematic illustration of a projection objective inaccordance with the invention in a meridional section;

FIG. 2 is an enlarged, not to-scale cutout from one of the mirrors in asectional view;

FIG. 3 is a flow chart for explanation of a first embodiment of theinvention;

FIG. 4 is a flow chart for explanation of a second embodiment of theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1 a microlithographic projection exposure apparatus isillustrated in a very schematized and not to-scale meridian section anddesignated in its entirety by the reference numeral 10. The projectionexposure apparatus 10 comprises an illumination system 12, in Which alight source 14 is arranged. The light source 14 serves to generateprojection light, Which is indicated by 16 and has a wavelength in theextreme ultraviolet spectral range, for example 13.5 nm In addition,illumination optics only indicated by 18 are part of the illuminationssystem 12 that allow to direct the projection light 16 onto a reticle20. The illumination device 12 is known as such in the prior art, forexample from EP 1 123 1 95 A1, and is therefore not described here inmore detail.

The projection light 16 reflected by the reticle 20 enters a projectionobjective 22, which in the embodiment illustrated, contains 6 asphericprojecting mirrors M1, M2, . . . , M6 arranged in a housing 23. Each ofthe mirrors M1 to M6 has a mirror support 241, 242, . . . 245 and/or246, onto which a coating 26 is applied. The coatings 26 can bedifferent from mirror to mirror and may comprise, for example,alternating successions of thin molybdenum and silicon layers. Suchlayered structures can reflect more than 60% of the impinging projectionlight 16. However, other laminated structures known in the prior art mayalso be used.

After reflection from the mirrors M1 to M6, the projection light 16impinges on a layer 28 that is deposited onto a silicon wafer 30 and issensitive to light of the particular wavelength. Thus a reduced image ofthe structures contained in the reticle 20 is produced on the layer 28.Since the fundamental arrangement of the mirrors M1 to M6 in theprojection objective 22 is known as such in the prior art, for examplefrom U.S. Pat. No. 6,353,470 B1, explanation of further details in thisregard is dispensed with.

The mirror supports 241 to 246 consist of a material that can beprocessed with high precision and preferably possesses low thermalcoefficients of expansion. Titanium-enriched quartz glass, for example,is possible in this context, and in particular ULE□, whereby ULE standsfor “ultra low expansion” and is a registered trademark of the CORNINGCompany in the USA. In addition, certain glass ceramics such as forinstance ZERODUR□ (registered trademark of the Schott Glass Company,Germany) are suitable material for the mirror supports 241 to 246.ZERODUR□ is a two-phase material having a crystalline phase thatcontracts with a rise in temperature and an amorphous phase that expandswith a rise in temperature. The temperature dependence of the thermalcoefficients of expansion can be very exactly adjusted by selecting themixing ratio of the two phases.

FIG. 2 shows a cutout from the last mirror M6 on the image-side in anenlarged, not to-scale sectional view. In the sectional view it can beseen that the coating 26, which is applied onto the optically activesurface 32 of the mirror support 246, consists of a plurality of thinsingle layers as described above. Real layer systems can containsubstantially more single layers than is illustrated in FIG. 2 forreasons of clarity.

A deformation on the surface 32 of the mirror support 246 is designatedby 34, which was produced by way of subsequent removal of material fromthe otherwise uniformly curved surface 32. The maximum depth □ of thesurface deformation 34 lies in the order of magnitude of severalangstroms or a few nanometers. The layers of the coating 26 track thepattern of the surface deformation 34 without substantially changingtheir thickness. Thus, the surface deformation 34 continues, at least toa certain extent, as far as the outermost layer 35 of the coating 26 andforms a recess 36 at the outer surface. The form of this recesssubstantially corresponds to the surface deformation 34.

Since the surface deformation 34 on the surface 32 of the mirror support246 transfers to the coating 26, the wavefront of the impingingprojection light 16 is systematically affected in its phase. The shapeof the surface deformation 34 is defined such that wavefront phasechanges caused in the coating 26 correct at least partly aberrations ofthe projection objective 22. Since no material is removed from thecoating 26 and thus all single layers also have their ideal thickness inthe proximity of the recess 36, the reflectivity of the portion of thecoating 26 above the surface deformation 34 is not changed.

In the following a first embodiment of a method to manufacture theprojection objective 22 illustrated in FIGS. 1 and 2 will be describedwith reference to the flow chart shown in FIG. 3:

First, in a step S11, the six mirror supports 241 to 246 are provided.

Then, in a step S12, the mirror supports 241 to 245 of the mirrors M1 toM5 are provided with the coating 26. The mirror support 246 remainsuncoated.

In a step S13 all mirror supports, i.e. both the coated mirror supports241 to 245 as well as the still uncoated mirror support 246, areassembled and adjusted into the projection objective 22.

In a step S14 certain optical properties of the projection objective 22are measured. The measurement in this case can be performed in the sameway, as is known for instance from U.S. Pat. No. 6,266,389 B1 that hasalready been mentioned above. In contrast to the process describedthere, however, a special measuring light is used having a wavelengththat is selected in such a manner that the still uncoated mirror support246 also reflects the measuring light. For this purpose measuring lightwith a wavelength of several hundred nanometers, e.g. 365 nm or 248 nm,may be used. The illumination device 12 is, during the measurement,interchanged with another illumination device that generates light withthe desired wavelength directs the projection light into the projectionobjective 22. With the aid of interferometric measuring instruments thatare known as such, the wavefront aberrations of the projection objective22 are measured.

The measured wavefront is compared in a further step S15 with idealvalues, as they have been determined previously for the design of theprojection objective 22. With regard to this determination it is to beconsidered that the measuring light has a longer wavelength than thelight used lateron during the exposure process. Although the opticalpath in the projection objective 22 is independent of the wavelength ofthe light reflected from the mirrors M1 to M6 in first approximation,the reflection on the mirrors M1 to M6 may be different for themeasuring light and for the EUV radiation. This is because the measuringlight does not penetrate the coating 26 of the mirrors M1 to M6, but isalmost totally reflected from their outer surface pointing outwards.Thus, the coated mirrors M1 to M5 appear translatorily slightly offsetwhen exposed to the measuring light.

In a step S16 a surface deformation 34 for the still uncoated mirrorsupport 246 is computed on the basis of the comparison between measureddata and ideal values according to step S15. This computation isperformed in such a manner that a pre-defined imaging property of theprojection objective is improved. Computations of this type are-known inthe prior art, see, for example, U.S. Pat. No. 6,266,281 B1 that hasalready been mentioned and whose full disclosure is incorporated byreference.

In a step S17 the surface deformation 34 defined in the step 516 isproduced in any known way, on the mirror support 246 that has beenpreviously dismounted from the projection objective 22. For thispurpose, material may locally be removed by reactive ion beam etching,for example.

The surface 32 of the mirror support 246 reprocessed in such a way isprovided with the coating 26 in a step S18. As a result, the mirror M6receives the structure shown in FIG. 2.

The coated mirror support 246 is subsequently re-assembled and adjustedinto the projection objective 22 in a step S19. After final fineadjustment using the projection light, the projection objective 22 iscompleted.

In modification of the afore-described manufacturing method, all mirrorsupports 241 to 246 are assembled and adjusted into the projectionobjective 22 without preceding coating (step S13). The step S12, inwhich at least some of the mirror supports 241 to 246 are coated, istherefore not necessary. In the step S18 not only the reprocessed mirrorsupport 246, but also all remaining mirror supports 241 to 245, whichhave not been reprocessed, are then coated. A disadvantage of thisvariant is that the mirror supports 241 to 245 that have not beenreprocessed have to be dismounted from the projection objective 22 afterthe initial assembly into objective 22 in the step S13, and thesessupports 241 to 245 have to be assembled again into the projectionobjective 22 afterwards.

On the other hand, this variant has the advantage that it is notnecessary to determine before the measurement of the projectionobjective 22 which mirror supports 241 to 246 should be reprocessed ontheir surface 32. Although there are generally certain mirrors insidethe projection objectives 22 that are particularly suitable forproducing surface deformations that correct aberrations, however,depending on the type of manufacturing defects it may sometimes beexpedient produce surface deformations 34 on other or even on all mirrorsupports 241 to 246.

In the following another embodiment of a method for manufacturing theprojection objective 22 shown in FIGS. 1 and 2 is described on the basisof the flow chart shown in FIG. 4.

The manufacturing process likewise begins with the provision of mirrorsupports 241 to 246 in a step S21.

While for the mirrors M1 to M5 in each case only a single mirror support241, 242, 243, 244 and/or 215 is made, for the mirror M6 two mirrorsupports 246 are made that are as identical as possible. In order to beable to distinguish between these, the first complete set of mirrorsupports 241, 246 is referred to as master mirror supports in thefollowing, while the additional one mirror support for the mirror M6 isreferred to as a copy of the master mirror support 246.

In a step S23 the coating 26 is applied onto each of the master mirrorsupports 241 to 246.

In a step S24 the master mirror supports 241 to 246 are assembled andadjusted into the projection objective 22.

In a step S25 the projection objective 22, completed to this extent, ismeasured in the same way as discussed above with reference to the stepS14. Alternatively, projection light may also be used as measuringlight, since all mirrors M1 to M6 are provided with the highlyreflective coating 26.

After a comparison of the measured data with corresponding ideal valuesin a step S26, a surface deformation 34 is defined in a step S27 so thatthe imaging properties of the projection objective 22 are improved.

This surface deformation 34, however, is now produced in a step S28 noton the master mirror support 246 of the mirror M6, but on the stilluncoated copy of this master mirror support 246. The copy reprocessed insuch a manner is then provided with the coating 26 in a step S29 so thatthe mirror M6 receives the configuration shown in FIG. 2.

In a step S30 the original mirror M6 with the master mirror support 246is replaced by the reprocessed and coated copy of this mirror support.The projection objective 22 completed after final adjustment now has amirror M6 with a surface deformation 34, as is illustrated in FIGS. 1and 2.

It is to be pointed out that although the afore-described invention isparticularly suitable for EUV projection objectives, in principle it mayalso be used for the manufacture of projection objectives that aredesigned for longer wavelengths of 365 nm, 248 nm, 193 nm or 157 nm, forexample.

1. A method of manufacturing a projection objective of amicrolithographic projection exposure apparatus having several opticalelements, wherein at least one optical element is a mirror comprising amirror support and a reflective coating applied thereon, said methodcomprising the following steps: a) providing a mirror support for the atleast one mirror; b) assembling and adjusting the optical elementsincluding the mirror support for the at least one mirror into a housingof the projection objective; c) obtaining measured data relating to atleast one image property of the projection objective using measuringlight; d) comparing the measured data with desired values; e) defining asurface deformation on the mirror support of the at least one mirrorsuch that an improvement of the at least one image property can beachieved; f) producing the surface deformation defined in step e) on thesurface of the mirror support; g) applying the coating onto the surface,which has been reprocessed in step f), of the mirror support; h)assembling the mirror support coated in step g) into the housing.
 2. Themethod of claim 1, wherein the mirror support reprocessed in step f) andcoated in step g) is an identical copy of the mirror support that isassembled into the projection objective in step b).
 3. The method ofclaim 1, wherein the ideal values are determined under consideration ofthe wavelength of the measuring light.
 4. The method of claim 1, whereinthe projection objective contains several mirrors.
 5. The method ofclaim 4, wherein at least one, but not all mirror supports are providedwith a coating before obtaining the measured data in step c).
 6. Themethod of claim 4, wherein surface deformations are produced accordingto step f) on more than one mirror support.
 7. The method of claim 4,wherein a surface deformation is produced according to step f) on onlyone mirror support.
 8. The method of claim 1, wherein the measuringlight has a wavelength that is different from the projection light forwhich the projection objective is designed.
 9. A method of manufacturinga projection objective of a microlithographic projection exposureapparatus having several optical elements, wherein at least one opticalelement is a mirror comprising a mirror support and a reflective coatingapplied thereon, said method comprising the following steps: a)providing a master mirror support for the at least one nirror; b)providing a substantially identical copy of the master mirror support;c) applying the coating onto the master mirror support; d) assemblingand adjusting the optical elements including the master mirror supportcoated in step c) into a housing of the projection objective; e)obtaining measured data relating to at least one image property of theprojection objective; f) comparing the measured data with ideal values;g) defining a surface deformation on the mirror support of the at leastone mirror such that an improvement of the at least one image propertycan be achieved; h) producing the surface deformation defined in step g)on the copy of the mirror support; i) applying the coating onto the copyof the mirror support reprocessed in step h); j) replacing the mirrorsupport assembled in step d) into the housing by its copy, coated instep i).
 10. The method of claim 9 wherein the projection objectivecontains several mirrors.
 11. The method of claim 10, wherein copies ofseveral mirror supports are provided according to step b), reprocessedaccording to step h), coated according to step i) and assembled into thehousing (23) according to step j).
 12. The method of claim 10, wherein acopy of only one mirror support is provided according to step b),reprocessed according to step h), coated according to step i) andassembled into the housing (23) according to step j).
 13. The methodclaim 9, wherein the surface deformation is produced by local removal ofmaterial.
 14. A projection objective for a microlithographic projectionexposure apparatus, comprising at least one mirror that has a mirrorsupport with a surface that has a local surface deformation forcorrecting at least one imaging aberration, a reflective coating appliedon the surface of the mirror support.
 15. The projection objective ofclaim 14, wherein the surface deformation is produced by local removalof material.
 16. A microlithographic projection exposure apparatuscomprising an illumination system for producing short-wave projectionlight and the projection objective of claim
 14. 17. Themicrolithographic projection exposure apparatus of claim 16, wherein theillumination system comprises a source for X-ray radiation.
 18. A methodof manufacturing of micro-structured components, comprising thefollowing steps: a) providing a support on which a photo-sensitive layeris applied; b) providing a reticle that contains to be imaged onto thelayer; c) projecting at least one part of the reticle onto the layerusing the projection objective of claim
 14. 19. A micro-structuredcomponent that is manufactured according to the process of claim 18.